According to a conventional method for fabricating a semiconductor device, a thin film of aluminum or an aluminum alloy such as aluminum/silicon or aluminum/silicon/copper is deposited upon a layer of insulating material which is formed over a semiconductor substrate. The metallization layer is then masked and etched to form an electrode or conductor pattern.
Plasma etching of aluminum and aluminum alloy film to form conductor patterns has been practiced for some time. In a typical plasma-etching process for such films, selected portions of the film are exposed to a chlorine-containing plasma such as carbon tetrachloride or boron trichloride to form volatile aluminum trichloride. The aluminum trichloride is then removed resulting in a pre-patterned metallization image.
The volatility of aluminum trichloride is, however, somewhat marginal. Therefore, a relatively large amount of aluminum trichloride residue remains on the surface of the semiconductor wafer following the plasma etching process. In addition to aluminum trichloride, reacted chlorine species that have been adsorbed onto the surface of the wafer are also present at the end of the etch.
Removal of such residue is usually at the expense of penetration into the wafer substrate. Since the chlorine residuals, particularly aluminum trichloride, are hygroscopic, they delequiesce to form aluminum hydroxide and hydrochloric acid. The hydrochloric acid promptly etches, or corrodes, the metallization pattern. This causes loss of line dimension and possible catastrophic failure.
The more typically utilized aluminum alloys, such as aluminum/silicon, and particularly aluminum/silicon/copper, demonstrate this phenomenon on a significantly larger scale. Since copper is a non-volatile species in a chlorine plasma, the copper filters down to the substrate as the etch progresses. The copper retains a larger quantity of chlorine either adsorbed to its surface or in the form of copper chloride. These copper-chlorine compounds form copper hydroxide and hydrochloric acid.
Prior methods for alleviating the above-discussed corrosion problem have been only marginally successful. The most common technique has been to follow the chlorine plasma etch of the metallization pattern with exposure to a fluorine-containing plasma. One such fluorine passivation technique is described in co-pending U.S. patent application, Ser. No. 172,745, filed July 28, 1980, now U.S. Pat. No. 4,325,984, and assigned to the same assignee as the present application.
While post-etch exposure to a fluorine-containing plasma does alleviate the corrosion problem to a certain extent either by a halogen exchange reaction (fluorine for chlorine) or by removal of the chlorine species by etching of the silicon substrate, it does suffer from drawbacks. If a halogen exchange reaction is the operating mechanism, it is, in all likelihood, relatively inefficient. Halogen atoms increase in size with increasing atomic number. Therefore, the smaller fluorine atoms are less likely to replace chlorine atoms by momentum transfer than are bromine or iodine atoms. Furthermore, halogen compounds are decreasingly reactive with increasing atomic number.
Attack of the substrate is not a particularly practical approach. Not only does such attack cause a higher step which must be covered with later films, but design rules and process parameters are such that active silicon device regions may be exposed to plasma. Such exposed areas are rapidly etched in a fluorine plasma resulting in device failure.